I am liking this approach. Two different current transformers, one differential transformer for the hot and neutral, a second current transformer for just the ground lead. If hot and neutral are mismatched by 6 mA, or if the ground lead carries 6 mA, a 3 pole relay or latching switch opens all three conductors. (UL might insist on breaking the hot-neutral before breaking the EGC which sounds more expensive.)

JR, I'm pretty sure that a single current transformer with all three wires (H-N-G) running though it would detect any external fault leakage, whether outgoing from the guitar's hot chassis, or incoming from a hot mic. At least that's how I'm drawing it and following the paths inside my head. Easy enough to try with a clamp ammeter and a few "leak" resistors.

JR, I'm pretty sure that a single current transformer with all three wires (H-N-G) running though it would detect any external fault leakage, whether outgoing from the guitar's hot chassis, or incoming from a hot mic. At least that's how I'm drawing it and following the paths inside my head. Easy enough to try with a clamp ammeter and a few "leak" resistors.

I thought about that before and yes, it should detect an external fault current very inexpensively but it might interfere with detecting an internal flaky guitar amp leaking hot to it's own EGC since they would still null out(?).

I think it needs to be two separate current transformers, but I am open for all suggestions.

I think it needs to be two separate current transformers, but I am open for all suggestions.

You could be right. There's a lot of different possible failure modes to consider. And throwing a possible RPBG outlet into the mix complicates things even further. But I'm a firm believer in the logic that if it CAN happen, then it WILL happen.

The other concern would be an amp without an egc or missing a ground pin with a fault causing it to have hot chassis being used with another grounded piece of gear also supplied by the "protective device"-if one CT is used a hot-egc fault will not be detected. A separate CT will trip on EGC current regardless of where it comes from.

I have to believe that adding another CT to an existing GFCI design should be a minimal cost, the 3 pole 20 amp relay is the tough part to get around as far as cost-but I can't figure a good way around that-Solid state might save cost, but I don't trust it for a safety disconnect.

What I am not familiar with is the internal testing modern GFCIs do-what makes them know not to rest if they are defective? That might throw a monkey wrench in things-I am guessing the UL guys will want to maintain that standard.

The other concern would be an amp without an egc or missing a ground pin with a fault causing it to have hot chassis being used with another grounded piece of gear also supplied by the "protective device"-if one CT is used a hot-egc fault will not be detected. A separate CT will trip on EGC current regardless of where it comes from.

Lets hope

Quote

I have to believe that adding another CT to an existing GFCI design should be a minimal cost,

I suspect every penny counts in these things.

Quote

the 3 pole 20 amp relay is the tough part to get around as far as cost-but I can't figure a good way around that-Solid state might save cost, but I don't trust it for a safety disconnect.

I'm with you.. I don't trust solid state for complete isolation.

I've looked at some patents from a guy with a few GFCI designs and he used a latching relay with two contacts in one... it seems a third contact is not huge.

I tried to explain the issues as I understand them to him, I expect him to know the cheapest way to do it. Problem still is that I don't see a market large enough to justify too much cost/effort.

Quote

What I am not familiar with is the internal testing modern GFCIs do-what makes them know not to rest if they are defective? That might throw a monkey wrench in things-I am guessing the UL guys will want to maintain that standard.

Another tidbit, I suspect the UL guys will be reluctant to give up ground bonding, but if they do they will probably want to delay releasing the ground bond until after the hot and neutral is already open.

Lets hope I get a serious answer... it would be nice to come up with an effective solution for this.

Another tidbit, I suspect the UL guys will be reluctant to give up ground bonding, but if they do they will probably want to delay releasing the ground bond until after the hot and neutral is already open.

I hope that UL will allow an exception for a EGC contact in the AC power, but if that's not possible then remember that a double-pole/mag-set reed relay in the guitar's signal cable would accomplish the same disconnect for the guitarist. However, it would NOT eliminate the shock hazard from someone touching the hot chassis of the guitar amp and a grounded object. So you would still need a standard GFCI powering the amp for that sort of fault protection.

....., but if that's not possible then remember that a double-pole/mag-set reed relay in the guitar's signal cable would accomplish the same disconnect for the guitarist. However, it would NOT eliminate the shock hazard from someone touching the hot chassis of the guitar amp ....

which could lead back to my idea of a D.I. based solution. Even though I use a mic for the guitar, I always have a free strip or three somewhere that could supply phantom power to a couple of "Guitar No Shock" boxes.I always use volt-alert, but in the heat and confusion, shit happens that I may not catch. I haven't killed anyone yet, and have no intention of ever letting it happen.

I have a friend who makes guitar pedals (amptweaker.com) and I still plan to ask him what he thinks. He's an actual design engineer and since his pedals have battery power he could probably add a latching relay protection inside a pedal, while he may also have a feel for how much (little) guitar players are willing to pay for the extra human safety.

which could lead back to my idea of a D.I. based solution. Even though I use a mic for the guitar, I always have a free strip or three somewhere that could supply phantom power to a couple of "Guitar No Shock" boxes.I always use volt-alert, but in the heat and confusion, shit happens that I may not catch. I haven't killed anyone yet, and have no intention of ever letting it happen.

Just to run out this hypothetical if you have a console grounded mic pointed at the guitar cabinet and the player with guitar in hand touches that mic for any reason he will be putting himself between two EGC systems and exposed to potential shock hazard.

I have a friend who makes guitar pedals (amptweaker.com) and I still plan to ask him what he thinks. He's an actual design engineer and since his pedals have battery power he could probably add a latching relay protection inside a pedal, while he may also have a feel for how much (little) guitar players are willing to pay for the extra human safety.

I need to ask him today, while hopefully he is busy with christmas sales.

JR

Exactly.... but my napkin design suggests that a current transformer just might have enough output current to open up a latching reed relay. And a permanent magnet on a push button could "reset" the latching relay if it trips. If that's indeed the case, then this device could fit in a plastic in-line box that connects between the guitar cable and the amplifier. If (and this is a big IF) there's enough current flow from the current sensing transformer to open up the latching relay without amplification, then there are no batteries required. I'm going to see if I can get a few current transformers and latching reed relays to play with.

However, if this was something that could be built and sold for $50 at a profit, would that be too much money? Or is $30 a more acceptable price? Just remember that for this to happen there has to be a certain amount of profit in building and selling it.